The present invention relates to a device for controlling the filling of a pipe being laid in a stretch of water, comprising:                an upstream section defining a water introduction inlet;        an injector for injecting a process fluid into the water introduced into the upstream section;        an intermediate section mounted downstream of the injector to receive the water containing process fluid;        a downstream section for conveying water to the fluid transport pipe being laid.        
Such a device is designed to be mounted at the lower end of a fluid transport pipe being laid, in order to filter and/or treat the water introduced into the fluid transport pipe while it is being laid.
The fluid transport pipe is, for example, a rigid pipe formed by an assembly of pipe sections welded successively on a laying barge (“J Lay” and “S Lay” laying methods) or unwound from a laying barge (“Reel Lay” laying method).
For various reasons, it may be advantageous to lay the pipe partially filled with water, in particular to stabilize the pipe on the seabed during the laying operation. The water filling is carried out via an orifice located on the lower end of the pipe laid on the seabed (a so-called “free flooding” method). In this case, it is often necessary to filter and/or treat the water that enters the pipe.
A device for controlling the filling of the pipe of the aforesaid type, designated by the term “Passive Flooding Unit” or “PFU” is known, for example, from WO 02/084160. Such a device is suitable for flooding the pipe in a passive manner, filtering the water introduced into the pipe and, in particular, treating it with a process fluid to prevent corrosion. The aforesaid devices generally comprise a reservoir of process fluid, which is of a finite volume.
Such a device works correctly when the laying barge floats on a stretch of water that is not very rough. However, when the stretch of water is rough, the pipe being laid is subjected to variations in height due to the local movements of the stretch of water.
When the upper end of the pipe goes down, an extra volume of water is drawn through the control device by a suction effect. While, when the pipe rises, the additional volume of water is forced back through the control device in the opposite sense.
This is not desirable insofar as the displaced water contains process fluid which may be expensive and/or environmentally harmful. In addition, this phenomenon of suction and discharge also occurs when laying of the pipe is interrupted. Since the duration of the interruption periods is not predictable, the required amount of process fluid is difficult to predict. The required amount is not simply proportional to the volume of the pipe.
Moreover, given the compressibility of water and the oscillations of the fluid transport pipe, resonance phenomena may occur. These phenomena cause a very significant suction of liquid, followed by an ejection which may damage the control device, and, in particular, the pumps of the equipment for injecting the process fluid. This resonance phenomenon makes the evaluation of the volume of process fluid required even more unreliable.
An object of the invention is to provide a device for controlling the filling of a fluid transport pipe while it is being laid in a stretch of water, and which allows the laying of the pipe by partially filling it with water coming from the stretch of water, wherein the device operates reliably even when the stretch of water is rough.
To this end, the subject-matter of the invention is to provide a device of the aforementioned type, characterized in that it comprises:                a liquid volume compensation reservoir, having a variable receiving volume for water discharged from the fluid transport pipe,        a connector to which are connected the intermediate section, the downstream section and the liquid volume compensation reservoir, the inlet cross-section of the liquid volume compensation reservoir taken at the connector being greater than 40 times the internal cross-section of the intermediate section taken at the connector.        
The device according to the invention may comprise one or more of the following characteristics taken individually or in any technically feasible combination:                the inlet cross-section of the liquid volume compensation reservoir at the connection is greater than 150 times the internal cross-section of the intermediate section taken at the connector;        the liquid volume compensation reservoir comprises a delimiting member to limit the receiving volume, and which may be displaced or deformed in order to modify the receiving volume, the delimiting member being, in particular, a piston or a membrane, a face of the delimiting member opposite the receiving volume being subjected to the hydrostatic pressure of the stretch of water;        the delimiting member is adapted to be displaced or deformed when the absolute value of the difference between the pressure in the receiving volume and the hydrostatic pressure in the stretch of water applied to the delimiting member is greater than or equal to 0.1 bar, advantageously greater than or equal to 0.05 bar, more advantageously greater than or equal to 0.01 bar;        the maximum receiving volume accommodated by the liquid volume compensation reservoir is greater than 100 liters;        the injector for injecting the process fluid comprises a pump discharging into the intermediate section;        the pump is a passive pump which may be actuated by the water introduced through the upstream section, or may be an active pump comprising at least one electric or hydraulic pump motor;        the injector for injecting the process fluid comprises a reservoir of process fluid, mounted upstream of the pump;        the process fluid contains a corrosion inhibitor and/or a biocide;        the device comprises a filter, inserted on the upstream section, upstream of the injector for injecting processing fluid;        the downstream section is formed of a flexible pipe.        
The invention also relates to a fluid conveying assembly, comprising:                a fluid transport pipe having a lower end located in the vicinity of the bottom of a stretch of water and an upper end;        a device as described above, wherein the downstream section is mounted at the lower end of the fluid transport pipe, wherein the upstream section opens into the stretch of water.        
The assembly according to the invention may comprise one or more of the following characteristics taken individually or in any technically feasible combination:                the lower end of the fluid transport pipe is provided with a section reducer defining a water inlet opening in the fluid transport pipe, the water inlet opening having a diameter smaller than the diameter of the fluid transport pipe, and wherein the downstream section opens into the water inlet opening;        the cross-section of the water inlet opening is less than 5% of the cross-section of the fluid transport pipe.        
The subject-matter of the invention also relates to a method of laying a fluid transport pipe in a stretch of water comprising the following steps:                lowering of an assembly as defined above in a stretch of water;        progressive filling of the fluid transport pipe with water from the stretch of water through the upstream section, the intermediate section, the connector and the downstream section, and injection of process fluid by the fluid injector into the water introduced into the transport pipe;        partial discharge of water from the fluid transport pipe through the downstream section;        filling the receiving volume of the liquid volume compensation reservoir with the water discharged from the fluid transport pipe.        